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Flight and crash

The aircraft, reg. F-OIQI, was a de Havilland Canada DHC-6 Twin Otter built in 1979. The airframe had been through 55,044 cycles and 30,833 airframe hours at the time of the crash.

The pilot was 53-year-old French Polynesian Michel Santeurenne. Air Moorea flights generally only required a single pilot, and on the 9th Santeurenne was flying the short hop without any other crewmembers. He had completed 3,514.5 hours of flight time, including 110.3 hours for Air Moorea since 14 May 2007. He had worked for Air Moorea for three months at the time of the accident.

The oft-traveled Moorea to Tahiti route is one of the shortest in the world – only a 7-minute flight on average – and is flown 40 to 50 times a day. On 9 August 2007, F-OIQI was the aircraft operating the short route. The aircraft took off without incident just past noon, and was climbing through 400 feet when, according to eyewitnesses, it suddenly nosed down without warning and drove into the ocean, killing all 20 on board – 19 passengers and the pilot, Santeurenne.

The (300 Series) Twin Otter is not normally equipped with a cockpit voice recorder or flight data recorder; however, Air Moorea had installed a CVR in all of their DHC-6 aircraft. The BEA called in a French ship – the Ile de Ré – which had a submersible that would be used to explore the crash site; however, it would not arrive for 13 days. When it arrived, the CVR (which was embedded inside the largely-intact tail section) was extracted, and the majority of the wreckage was recovered with the assistance of a remote controlled submersible.

One possibility the investigation discussed was the possibility of engine failure as some eyewitnesses had claimed that the engines had failed after takeoff; however the CVR recording disproved the theory as the sound of the two Pratt & Whitney Canada PT6 propellers could be heard up until the recorder cut off on impact with the water. In addition, the prospect of a cockpit intrusion and/or hijacking was also brought up, but the CVR tape indicated nobody other than the pilot had been in the cockpit at any time in the flight.

However, the CVR tape did bring to light one piece of evidence – around 400 feet, just after the flaps were retracted, Santeurenne made a puzzled and quick exclamation, after which the aircraft drove into the sea. This hinted to investigators that the problem was sudden and without warning, as the fatal dive the aircraft entered had been spontaneous, giving Santeurenne no warning.

In addition, wreckage recovered by the secondary recovery effort provided another massive clue. The DHC-6's rear control surfaces are linked to the cockpit by four separate cables – two pitch control cables (up and down elevator positions) and two yaw control cables (left and right rudder positions). Each cable is made of bundles of metal wire braided around each other for added strength. The four cables had been recovered from the wreckage. Three of the cables, including both rudder cables and the up-elevator pitch cables, showed telltale signs of impact breakage. They had snapped unevenly and the edges were very frayed, showing that they had snapped and unraveled due to the force of the impact with the water. The fourth cable – the other pitch control cables that moved the elevator down – told a different story. It was not broken in the same manner as the other three; instead, it was far shorter, still tightly braided, and had no signs of impact damage; evidently it had fractured fairly cleanly.

The cable, which was found, had been rubbing against the rope guide that ran from the cockpit to the tail section. The constant wear was exacerbated by the pressure put on the cable, which was moved hundreds of times a day on the DHC-6's short flight from Moorea to Tahiti and back. When the cable snapped on the fateful flight, the elevator immediately jammed in a downward position, sending the aircraft into an unrecoverable dive. The investigators concluded Santeurenne had no chance to save the plane, as the aircraft was only at 400 feet when the cable snapped.

In addition, F-OIQI was the only airplane within the fleet of Air Moorea equipped with stainless steel elevator cables, while the other aircraft were equipped with carbon steel elevator cables. The two types of cables are interchangeable and have the same maintenance intervals and activities. The operator did not know however, that the carbon cables are more susceptible to corrosion, while the stainless steel cables are more susceptible of wear. The maintenance interval is based on calendar intervals and does not take the activity of the airplane into account. Many operators had therefore reduced inspection intervals on their own, meaning a worn cable would likely not be found until it was too late.

But pressing questions remained, as a lab experiment carried out by the BEA showed that the cable, albeit very worn down, should have been able to withstand the strain of the various elevator inputs. The analysis found, that although the cable showed traces of severe chafing and wear (which were not noticed during initial visual inspection by BEA), the rupture could not have occurred without additional beforehand weakening of the cable by an extreme force exceeding the possibilities of pilot input or regular operating conditions. Investigators were puzzled by this. If the cable had been worn but could still have withstood the strain of a seven-minute flight, there was no obvious reason for why it snapped without warning, sending the plane into its fatal dive.

However, an important piece of evidence soon came to light – a combination of a dangerous airline practice and an airport design flaw. A system for locking the flight controls is available on the DHC-6, when the airplane is parked, to avoid flight control surface flapping under the effect of wind gusts. The rudder is locked in a neutral position by centering the rudder pedals. The locking is done at the level of the rudder control crank under the cockpit floor. The lock control is held in vertical position by a spring pin situated at the lower end of an upright linked to the lock for the aileron and elevator controls. The aileron and elevator flight control surfaces are locked by means of a device that links the pilot wheel and column to the structure of the flight instrument panel. Originally, the elevator was locked in a neutral position. However, with some flights having taken place with the lock system in place, the manufacturer modified the system so that the elevator control was maintained in maximum forward position (elevator fully pitch-down position). As the locking of the flight controls is done in the cockpit, all of the linkages made up of cables and pulleys as well as the mobile flight control surfaces are subject to loads induced by air blast on the flight control surfaces. The Air Moorea Operations Manual in the section "2- NORMAL PROCEDURES, 2.1-Normal Check-list" specifies locking the controls when there is a prolonged stopover. This is specially the case during parking overnight at Tahiti Faa'a.

At Fa'a'ā International Airport, the parking area for small craft such as the DHC-6 is just aside of the parking area for larger aircraft such as the A340-300, which Air France and Air Tahiti Nui often use on a flight from Paris to Faa'a. If the jet had been pushed back at a certain angle, the jet blast from the engines would have been straight into the parked Twin Otters. However, a jet blast barrier theoretically should have been in place to shield the small aircraft from this exhaust. Indeed, before 2004, jet blast barriers did protect the small-craft parking ramp from the effects of jet blast from heavy aircraft pushed back from the B1 parking ramp. However, the entry into service of the L taxiway, at the end of 2004, was accompanied by the removal of the barriers. This removal left the smaller propeller airplanes directly in the path of the jet blast from large jets pushing back from the B1 parking ramp.

The BEA concluded that an Airbus A340, departing the stand at Faa'a (with F-OIQI being parked overnight about 80 meters away), produced a sufficient blast cause the breakage of several strands of the cable. The upper part of the elevator, which was locked in a down-position, was exposed directly to the blast from the A340's engines, forcing it to bend downwards past its normal limits. This weakened the cable that was holding the elevator in the down position. The remaining strands wore further down through normal flight loads, and finally broke suddenly during the regular control forces on 9 August 2007. When Santeurenne retracted the flaps at around 400 feet, it caused the nose to go sightly down. When Santurenne attempted to bring the nose up again, the strained cable snapped, sending the plane into its unrecoverable, fatal dive.

The BEA concluded that the most probable cause of the crash was the loss of pitch control after the elevator cable broke just after the retraction of the flaps. The report said that the failure was caused by the sequence of following events:

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